This invention relates to electrolytes for redox flow batteries, and more particularly to an electrolyte for a redox flow battery wherein the cell resistivity is reduced and wherein the solubility of an active material is improved.
Electric power is readily convertible into various forms of energy and easy to control, and causes no environmental pollution during its consumption. Accordingly, the proportion of electric power in the total energy consumption is increasing every year. A unique feature of electric power supply is that it is produced and consumed, simultaneously. Under this condition, the electric power industries are required to supply electricity of high quality of constant frequency and constant voltage with high reliability while responsively meeting the demand of electricity, changing every moment. In actual practice, nuclear power plant and large-scale thermal power plants which cannot readily change their output but have high efficiency are operated at the rated output of as high efficiency as possible for base load. The large increase in demand for electric power in the daytime, i.e. peak load, is met by hydroelectric power generation suitable for changing its load according to electricity demand.
The surplus power in the nighttime generated by nuclear power and large-scale thermal power plants of good economy is stored by pumped hydro. However, the number of economically feasible sites for pumped hydro are becoming gradually reduced. Under such circumstances, various types of secondary batteries for battery energy storage systems which have relatively less restriction for location and cause no environmental pollution, and wherein electric power, which is an energy having great flexibility, is stored have been proposed. Among these secondary batteries, redox flow batteries wherein two redox systems are brought into contact with each other via a membrane are promising batteries.
By the term redox flow battery as used herein is meant a secondary battery which has flow cells, tanks and pumps, two electrolytes comprising aqueous solutions of active materials which are fed to the flow cells, and in which electrochemical reactions of charge and discharge are carried out, and electricity being stored in the tanks or the like in the form of a change in valence of the redox ions dissolved in the aqueous solution.
By the term Redox system is meant chemical species which can exists in different oxidation states or ion valences such as Fe.sup.3+ /Fe.sup.2+ and Cr.sup.3+ /Cr.sup.2+. The development of a redox flow battery comprising iron and chromium ions wherein an Fe.sup.3+ /Fe.sup.2+ hydrochloric acid solution is used as a positive electrolyte and a Cr.sup.2+ /Cr.sup.3+ hydrochloric acid solution is used as a negative electrolyte is currently well in progress. Other systems known include redox systems comprising iron-titanium, manganese-chromium, bromine-chromium, chlorine-chromium and the like.
The redox flow battery is superior to conventional secondary batteries in the following respects: (1) The storage capacity can be selected according to its use and location by merely varying the tank size; and (2) it is suitable for long term storage of weekly cycle or the like. Thus, the redox flow battery is a noteworthy battery for electric power storage.
FIG. 1 shows the outline of the redox flow battery. Power generated in a power station 1 is transmitted to a transforming station 2, from which power of transformed voltage is supplied to a load 3 and to a rectifier-inverter 4. The rectifier-inverter 4, converting AC from transformation station 2 into DC, supplies DC power to the redox flow battery 5 during charge and, inverting DC from redox flow battery 5, supplies AC power to transforming station 2 during discharge. The battery 5 consists of flow cell 8, tanks 6 and 7, pumps 13a and 13b.
The inside of the flow cell 8 is divided by a partitioning membrane 9 into positive electrolyte chamber 10a and negative electrolyte chamber 10b, which consist of the positive electrode 11 and the negative electrode 12, respectively. Positive electrolyte such as hydrochloric acid solution containing Fe.sup.2+ /Fe.sup.3+ ions and negative electrolyte such as hydrochloric acid solution containing Cr.sup.3+ /Cr.sup.2+ ions, stored in tanks 6 and 7, respectively are circulated during charge or discharge with pumps 13a and 13b through electrolyte chambers 10a and 10b via passages 14 and 15, respectively. For example, when an electrolyte containing Fe ions is used as a positive electrolyte and an electrolyte containing Cr ions is used as a negative electrolyte, the electrochemical reactions shown in the following formulae (1), (2) and (3) occur at each electrode surface in the flow cell 8 during charge or discharge. EQU Positive Electrode:Fe.sup.3+ +edischarge.revreaction.chargeFe.sup.2+( 1) EQU Negative Electrode:Cr.sup.2+ discharge.revreaction.chargeCr.sup.3+ +e (2) EQU Total Reaction:Fe.sup.3+ +Cr.sup.2+ discharge.revreaction.charge Fe.sup.2+ +Cr.sup.3+ ( 3)
Thus, electric power is stored in each electrolyte, namely, in the active materials. Accordingly, when the concentration of the active material (for example, iron ions in the positive electrode and chromium ions in the negative electrode) dissolved in the electrolyte is lower, a larger amount of electrolyte and a higher flow rate of the recycled electrolyte are required in order to store the same amount of electricity. Accordingly, the capacity of tank or pump must be increased. In order to reduce these disadvantages, it is desirable to increase the concentration of the active material as high as possible. On the other hand, in order to reduce the cell resistivity, it is desirable that the hydrogen ion concentration of the electrolyte be increased and that the resistance of the ionic conduction through the membrane be reduced.
Heretofore, hydrochloric acid solutions have been used as the electrolytes, as described above. When the concentration of the hydrochloric acid is greatly increased, the hydrogen ion concentration is increased, and the resistance of the membrane is reduced. However, the chloride ion concentration is also increased at the same time, and therefore the solubility of chromium chloride which is an active material is reduced by common ion effect of the chloride ion. Thus, it is believed that the concentration of hydrochloric acid is suitably from 3 to 4 normal (N).
An electrolyte containing sulfuric acid, wherein the hydrogen ion concentration is increased without increasing the chloride ion concentration, and thereby the cell resistivity is reduced without reducing the solubility of the active materials has been considered.
The cell resistivity of the redox flow battery primarily comprises electric resistance due to the overvoltage of the electrode reaction, and the above mentioned resistance of the membrane.
As described above, the addition of sulfuric acid can increase the hydrogen ion concentration and can reduce the electric resistance of the membrane. However, the portion of the resistance due to the electrode reactions increases with increasing concentration of the sulfuric acid. Thus, the overall battery performance is less improved (see, D through F of Example 2, (ii) of Example 4 and (ii) of Example 5 described hereinafter).